Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS) and a plurality of subscriber units. An established harmonised cellular radio communication system, providing predominantly speech and short-data communication, is the Global System for Mobile Communications (GSM). GSM is often referred to as 2nd generation cellular technology.
An enhancement to this cellular technology, termed the General Packet Radio System (GPRS), has been developed. GPRS provides packet switched technology on GSM's switched-circuit cellular platform. A yet further enhancement to GSM that has been developed to improve system capacity can be found in the recently standardised Enhanced Data Rate for Global Evolution (EDGE) that encompasses Enhanced GPRS (EGPRS). A still yet further harmonised wireless communication system currently being defined is the universal mobile telecommunication system (UMTS). UMTS is intended to provide a harmonised standard under which cellular radio communication networks and systems will provide enhanced levels of interfacing and compatibility with many other types of communication systems and networks, including fixed communication systems such as the Internet. Due to this increased complexity, as well as the features and services that it supports, UMTS is often referred to as a third generation (3G) cellular communication technology. In UMTS subscriber units are often referred to as user equipment (UE).
Within GSM, two modes of operation (e.g. two modulation schemes) may be used, Gaussian Minimum Shift-keyed (GMSK) modulation and 8-phase shift keyed (8-PSK) modulation. GMSK is a constant amplitude phase modulation scheme whilst, for the second generation (2G) standard, 8-PSK incorporates both amplitude and phase modulation.
In such cellular wireless communication systems, each BTS has associated with it a particular geographical coverage area (or cell). The coverage area is defined by a particular range over which the BTS can maintain acceptable communications with subscriber units operating within its serving cell. Often these cells combine to produce an extensive coverage area.
Wireless communication systems are distinguished over fixed communication systems, such as the public switched telephone network (PSTN), principally in that mobile stations/subscriber equipment move between coverage areas served by different BTS (and/or different service providers). In doing so, the mobile stations/subscriber equipment encounter varying radio propagation environments. In particular, in a mobile communication context, a received signal level can vary rapidly due to multipath and fading effects.
One feature associated with most present day wireless communication systems allows the transceivers in either or both the base station and/or subscriber unit to adjust their transmission output power to take into account the geographical distance between them. The closer the subscriber unit is to the BTS's transceiver, the less power the subscriber unit and BTS's transceiver are required to transmit, for the transmitted signal to be adequately received and decoded by the other unit. Thus, the transmit power is typically controlled, i.e. set to a level that enables the received signal to be adequately decoded, yet reduced to minimize potential radio frequency (RF) interference. This ‘power control’ feature saves battery power in the subscriber unit. Initial power settings for the subscriber unit, along with other control information, are set by the information provided on a beacon (control) physical channel for a particular cell.
Furthermore, in a number of wireless communication systems, the effect of fast fading in the communication channel is a known and generally undesirable phenomenon caused by the signal arriving at a receiver via a number of different paths. Therefore, fast power control loops are often adopted to rapidly determine and optimize the respective transmit power level.
It is known that within the field of power control techniques that the power control loop gain and bandwidth of existing power control mechanisms are severely limited by the loop latency (also known as dead time or pure lag). As loop latency increases, the controller gain has to be reduced to maintain adequate stability margins. As a consequence the closed loop bandwidth will diminish with the system becoming progressively slower and more sensitive to variations in internal dynamics. As a result of these variations, critical standards' test specifications are failed, such as:
(i) Power versus time (PvT), or
(ii) Out-of-band spectral emission performance
Furthermore, any variation in the threshold or activity level is known to complicate the loop ramp-up sequencing.
The conventional solution to the above problems has been to perform extensive factory calibration or ‘phasing’ of the loop, where the controller settings are phased with target power and frequency. However, such a solution is not a viable option in the future due to the unacceptable overhead that such a factory-tuning exercise creates in the manufacture of a mass-produced product, such as a 3G cellular phone. Furthermore, this solution offers no robustness with regard to temperature and power supply variations that are inherent within an operational unit.
A need therefore exists, in general, for an improved power control arrangement and method of operation, wherein the abovementioned disadvantages may be alleviated.